The invention relates to a knee prosthesis comprising a medial condyle, the running surface of which and the counter-surface A of which that lies on a tibia platform are sections of spherical surfaces with a radius R1, the center M1xe2x80x2 of which forms in its vertical projection onto the counter-surface A an axis with a point M1 about which a lateral condyle is pivotal.
A knee prosthesis is shown in the patent specification U.S. Pat. No. 5,219,362 in which a large contact surface exists on the medial side between the medial condyle and the oppositely lying tibia platform. In the lateral condyle there is at most a linear contact during the flexion, which leads to undesirably high area pressings.
It is an object of the invention to improve this situation. This is achieved in that the lateral condyle with its running surface and its counter-surface B are likewise sections of spherical surfaces with a radius R2 with center M2xe2x80x2, the vertical projection of which onto the counter-surface B corresponds to a point M2; and in that the counter-surface B belongs to an artificial meniscus part which can be displaced on a plane of the tibia platform and which, guided on a circular arc, is pivotal with the point M2 about the point M1 at a distance R3 which corresponds to the distance a of the centers M1xe2x80x2, M2xe2x80x2.
This arrangement prevents large surface pressings and allows the tibia a rotational movement of the tibia platform about a point M2 on its medial counter-surface A during the flexion corresponding to the attached ligaments and, with the rotational movement, a movement of the lateral side of the tibia platform towards the anterior, whereas the meniscus part of the lateral side remains in place relative to the lateral condyle.
The tibia platform is pivotal from a middle position which corresponds to the extension position and in which the points M1, M1xe2x80x2, M2, M2xe2x80x2 lie in a transversal plane of the tibia platform by an angle xcex11xe2x89xa70 towards the posterior into a position which corresponds to the hyperextension and in the event of complete flexion by an angle xcex12 between 5xc2x0 and 20xc2x0 towards the anterior. Furthermore, it can be advantageous to limit the sum of the angles xcex11 and xcex12 to a value between 12xc2x0 and 18xc2x0 in order constructionally to achieve a larger counter-surface B for the lateral condyles.
A circular-arc-shaped bulge of the tibia platform, which projects over a sliding plane for the meniscus part and,engages into a groove of the meniscus part for the guiding about the point M1, has the advantage that no foreign objects can collect and be squashed on the sliding plane or in the groove since they are always brushed off again through the relative movement between the meniscus part and the tibia platform. This presupposes that no larger shoulders, but rather at most end abutments for the movement, project above the sliding plane in the direction of movement.
The tibia platform itself can however also be provided on the lateral side with a wide circular groove, the limiting radii of which have the common center M1 on the medial side; and the meniscus part is designed as a part of a circular ring which is guided by the groove.
Through the spherical contact surface on the medial side and through the spherical contact surface of the lateral meniscus part, which is planar on the lower side, the surface pressing can be lowered to such an extent that a plastic body is inserted medially on the tibia platform and that a plastic body is inserted as meniscus part, both for example of high molecular polyethylene.
The use of plastic bodies medially and laterally has the advantage that the tibia platform can be made symmetrical with respect to its sagittal plane and that the tibia platform can be inserted selectively for a left or right knee in that the medial plastic body can be firmly anchored and the lateral meniscus part is pivotal with its point M1 on a radius R3 about the point M1 of the medial side.
In the following the invention will be explained with reference to exemplary embodiments.